1. Technical Field
Methods of manufacturing a flash memory devices are disclosed, and more particularly, a manufacturing method is disclosed that is capable of stabilizing a threshold voltage of a flash memory device which uses a high voltage.
2. Discussion of Related Art
Recently, for an implementation of a flash memory device, a self-aligned shallow trench isolation (SA-STI) has been adapted to form device isolation films to prevent damages on a tunnel oxide film and improve poor device characteristics. Meanwhile, for applying a high voltage to a well area and a junction area of the transistors which are formed according to the aforementioned method, the junction between a source and a drain has been formed as a double doped drain (DDD) junction instead of a pulse junction by using a plug implantation method. However, the number of the DDD junctions is inevitably limited to improve a breakdown voltage in preparation for application of a high voltage. In this case, the lowered ion concentration in the source area and the drain area makes an operating voltage of 1.0V or less which is normally used in a typical transistor appear to be relatively high. In addition, with a p-type dopant injected by the ion implantation to control a threshold voltage of a channel area, it is difficult to obtain an operating voltage less than 1.0V even by the implantation with a minimum quantity of ions. Typically, in order to improve a breakdown voltage, the ion implantation is performed with B11 ions instead of BF2 ions, which have a possibility of remnants in the junction area. However, even if the ion implantation is performed with boron B11, the boron may react in the subsequent heat treatments so that the injected dopant may be susceptible to a transient enhanced diffusion (TED).